Integrated inductor

ABSTRACT

The invention concerns an intgrated inductor ( 20 ), consisting of a flat winding of one or several turns ( 21, 22, 23 ) made of a conductive material above a substrate provided with at least a subjacent conductive level wherein is produced, through a contact pick-up strip ( 12′ ), at least an intersection of the winding, the width of at least one turn and/or one interval between two turns being reduced in line with said contact pick-up strip.

The present invention relates to the field of integrated circuits, andmore specifically to the manufacturing of an inductance formed above asemiconductor substrate.

FIGS. 1A and 1B show, respectively in a top view and in a cross-sectionview along line B-B′ of FIG. 1A, a conventional example of an inductance1 formed above a semiconductor substrate 2. Inductance 1 includes anumber of generally concentric turns or spirals (at least one spiral)obtained by the deposition of a conductive element on an insulatinglayer 3 (FIG. 1B). Insulating layer 3, for example, silicon oxide, restson the last metallization level 4 added on substrate 2 after forming ofintegrated components in this substrate. In the example of FIG. 1B, twoother metallization levels 5, 6 have been illustrated in dotted linesbetween substrate 2 and upper level 4. Each level is of course separatedfrom the underlying level by an insulating layer, respectively 7, 8. Theconductive element of inductance 1 is conventionally of constant widthand thickness. It is deposited on insulating layer 3, in the form of aflat winding from a first internal end 10 to a second external end 11.

To enable connection of inductance 1 to the rest of the integratedcircuit or to a terminal of a package, it is necessary to provide acontact recovery from internal end 10 of the winding to the outside ofthis winding. Conventionally, this contact recovery is obtained by usingan underlying metallization level (generally, upper level 4). Aconductive track 12 (generally rectilinear) is formed therein betweenthe location under internal end 10 of inductance 1 and the locationunder a pad 13 outside of the winding. Pad 13 is formed on insulatinglayer 3 in the same conductive material as the winding of inductance 1.Vias 14 and 15 (for example, made of tungsten) electrically connect end10 and pad 13 to the respective ends of underlying track 12.

In the forming of an inductance, its quality factor is generally desiredto be optimized. Among the parameters having an influence upon thisquality factor, the parasitic resistance (series resistance) is a majorparameter. To reduce the resistance of the conductive element againstthe flowing of current, its section is generally desired to bemaximized. Not only the thickness, but also the width of the spirals ofinductance 1 are then increased.

A disadvantage is that the contact recovery from the internal end of thewinding introduces a series resistance that annuls the beneficialeffects of the section increase of this winding. Indeed, the thicknessof the metal levels underlying the winding is imposed by the technologyin which the other components integrated with the inductance aremanufactured.

For example, the metallization levels are formed in aluminum depositedover a thickness from 0.8 to 1 μm. The conductive level added on top ofthe structure and in which the spirals are formed has, in the case ofaluminum, a thickness on the order of 2.5 μm. However, to avoidadversely affecting the integrated circuit manufacturing, such athickness increase is only conceivable on the last deposited level.

This problem is posed whatever the number of spirals of the inductanceand whatever the conductive materials used. Further, it is moregenerally encountered each time a crossing is desired to be made in awinding of an integrated inductance.

The present invention aims at providing a novel integrated circuitinductance that overcomes the disadvantages of known inductances.

The present invention more specifically aims at solving the problemsassociated with the contact recovery from the internal end of theinductance.

More generally, the present invention aims at providing a solution tothe problem of crossing, by contact recovery in a lower level, of a flatwinding of an inductance.

To achieve these objects, the present invention provides an integratedinductance, formed of a flat winding of at least one spiral made of asemiconductor material above a substrate provided with at least oneunderlying conductive level, in which the winding is crossed at leastonce by a contact recovery track, the spiral width being reduced abovesaid contact recovery track.

The present invention also provides an integrated inductance, formed ofa winding of several spirals, the width of at least one spiral and/or ofat least one interval separating two spirals being reduced above saidcontact recovery track.

According to an embodiment of the present invention, the crossing isused for the contact recovery from an internal end of the winding to anexternal pad.

According to an embodiment of the present invention, the pattern of thespirals is such that the external spiral is, at the level of the contactrecovery, closer to the center of the winding than the rest of thisexternal spiral.

According to an embodiment of the present invention, the resistance persquare of the conductive material constitutive of the winding issubstantially smaller than the resistance per square of the underlyingconductive level in which is formed the contact recovery, the thicknessof the conductive material constitutive of the winding being,preferably, substantially greater than the thickness of the underlyingconductive level.

According to an embodiment of the present invention, the intervals, inthe contact recovery alignment, between the two connected windingportions, are minimized.

According to an embodiment of the present invention, the length of thenarrowed section(s), which depends on the width of the contact recoverytrack, is chosen to be as short as possible.

According to an embodiment of the present invention, the conductivematerial is aluminum, the underlying conductive level being also made ofaluminum.

According to an embodiment of the present invention, said conductivematerial is copper having a thickness of several tens of micrometers,the underlying conductive level being made of aluminum of a thickness onthe order of one micrometer.

According to an embodiment of the present invention, said conductivelevel is formed by the upper metallization level used for theinterconnections of other components of the integrated circuit.

The foregoing objects, features and advantages of the present inventionwill be discussed in detail in the following non-limiting description ofspecific embodiments in connection with the accompanying drawings.

FIGS. 1A and 1B, previously described, respectively show in a top viewand in a cross-section view a conventional example of an integratedcircuit inductance;

FIGS. 2A and 2B show, respectively in a top view and in a cross-sectionview, an embodiment of an integrated inductance according to the presentinvention; and

FIGS. 3A and 3B illustrate, by representations of an integratedinductance, respective in top view and in cross-section view,alternative embodiments of the present invention.

The same elements have been designated with the same references in thedifferent drawings. For clarity, only those elements that are necessaryto the understanding of the present invention have been shown in thedrawings and will be described hereafter. In particular, the componentswith which one or several inductances are integrated on thesemiconductor substrate have not been shown in the drawings and are noobject of the present invention.

A feature of the present invention is to provide a narrowing of theconductive element constitutive of a winding of an integratedinductance, above an underlying conductive track enabling, by a contactrecovery, crossing of the winding. Such a localized narrowing of thewinding spiral(s) enables reducing the length of the underlying contactrecovery section, and thus the series resistance of the inductance. Inan inductance with several spirals, a feature of the present inventionis to provide, above the underlying contact recovery, a narrowing of atleast one spiral of the conductive element and/or of at least oneinsulating interval between spirals.

The present invention will be described hereafter in relation withexamples of contact recovery of an internal end of the winding. However,all that will be discussed hereafter more generally applies to acrossing at any point of the winding.

FIGS. 2A and 2B show, respectively in a top view and in a cross-sectionview along line B-B′ of FIG. 2A, an embodiment of an integratedinductance according to the present invention.

Conventionally, an inductance 20 according to the present invention isformed of one or several spirals of a conductive element deposited abovea semiconductor substrate 2 in which integrated circuits have beenformed. Inductance 20 is deposited flat on an insulating layer 3covering the last metallization level 4 of the integrated circuit. Theexample of FIG. 2B shows the same metal level 5, 6 and insulating levels7, 8 as in the example previously described in relation with FIG. 1B. Inthe example of FIG. 2A, inductance 20 includes three and one quarterspirals of square shape. However, an inductance according to the presentinvention may have any shape (round, oval, or polygonal) and also anynumber of spirals. The use of rectilinear sections however simplifiesthe manufacturing.

As previously, a contact recovery is provided from internal end 10 ofthe winding to a pad 13 external to this winding. This contact recoveryis performed by means of a conductive track 12′ obtained, for example,in the last metallization 4 underlying the conductive elementconstitutive of inductance 20. Internal end 10 and pad 13 are connectedto the respective ends of track 12′ by means of vias 14, 15.

As an alternative, in a circuit with several metallization levels,several superposed tracks may be used in the successive metallizationlevels to form the contact recovery. These tracks are then connected inparallel by means of vias crossing the different insulating layers 3, 4,and 8. Indeed, these metallization levels are available since theinductance is generally placed on a portion of the substrate containingno other component. However, this solution leads to using levels closerand closer to the substrate, which increases stray capacitances betweenthe winding and the substrate. The choice of this alternative depends onthe desired compromise between the decrease of the series resistance andthe increase of such capacitances. According to another alternative, thefirst metallization level used will not be the level closest to thewinding. In this case, the stray capacitance between the winding and thecontact recovery is decreased.

According to the embodiment shown in FIGS. 2A and 2B, each spiral 21,22, or 23 that must run above contact recovery track 12′ exhibits, abovesaid track 12′, a narrowed section, respectively 21′, 22′, and 23′. Asseen from above, narrowing 22′ of intermediary spiral 22 is, forexample, aligned with the rest of the rectilinear section in which it isformed. Narrowings 21′ and 23′ are then not aligned with the rest of thecorresponding sections of spirals 21 and 23 to bring sections 21′ and23′ as close as possible to section 22′. The connection between eachnarrowed section and the rest of the corresponding spiral may have anyshape (for example, oblique, as shown, or with a right angle). Thenarrowings enable reducing the length of track 12′ with respect to asame track which would have to cross sections 21, 22, and 23 in theirnon-narrowed portions. For spirals with rectilinear sections, thenarrowed sections are parallel to one another and, for example,perpendicular to the contact recovery track, the length of which isdesired to be minimized. Accordingly, a contact recovery sectionaccording to the present invention has a resistance smaller than that ofa conventional section in the same technology. By reducing theresistance of the contact recovery section, the general seriesresistance of the inductance is decreased and its quality factor is thusincreased.

It could have been thought that by narrowing the conductive spirals, theseries resistance of the corresponding sections is increased in such away that the length decrease of the contact recovery track is useless.However, this does not occur. First, the narrowing provided by thepresent invention is localized and the shortest possible to minimize theresistance introduced in each spiral. Further, this narrowing does notgo along with a thinning down, so that the cross-section of sections21′, 22′, and 23′ remain relatively large (especially as compared withthe underlying level). Moreover, integrated inductances are generallyused for high-frequency applications where the current in the inductanceis essentially a function of the perimeter of its cross-section (skineffect). Accordingly, if the spirals are sufficiently thick (thickerthan they are wide), the inductance is not debased by the narrowingsprovided by the present invention.

Taking the preceding example of an aluminum conductive element of a2.5μm thickness placed on a stacking of aluminum metallization levels of0.8 μm, a narrowing down to a 2μm width (if allowed by the seriesresistance then introduced) can be provided in spirals having for therest a given width ranging, for example, between one and a few tens ofμm.

In an alternative (not shown) where only the intervals between spiralsare narrowed above the contact recovery, the series resistance of thewinding is not modified.

The minimum width of the conductive sections of the inductance and theintervals between spirals is essentially linked to the technologicalprocess used according to the thickness of these conductive sections.

For example, to improve the conductivity of the inductance, copper,which can then be deposited in a much thicker layer to form theinductance, is also used. Copper thicknesses of several tens of μm (forexample, approximately 30 μm) can then be obtained. With such atechnology, the minimum width and the spacing are approximately half thethickness (that is, for example, approximately 15 μm). The width of thenon-narrowed sections is for example on the order of 30 to 40 μm. Theunderlying metallization levels remain, for example, made of aluminum.

Of course, the nature of the material and/or its thickness may bemodified. What matters is that the resistance per square of theconductive element of the winding is smaller than the resistance persquare of the metallization level containing the contact recovery.

FIGS. 3A and 3B illustrates alternative embodiments of an inductanceaccording to the present invention. FIG. 3A is a top view and FIG. 3B isa cross-section view along line B-B′ of FIG. 3A.

The embodiment illustrated in FIGS. 3A and 3B is a hexagonal inductance30 having four spirals and one third, formed of rectilinear sections. Inaddition to the shape variation, this embodiment includes anothervariation in the arrangement of narrowings 31′, 32′, 33′, and 34′ ofspirals 31, 32, 33, and 34 above track 12′ of transfer of internal end10 of inductance 30 to external pad 13. In the embodiment of FIGS. 3Aand 3B, these narrowings are brought as close as possible to internalend 10 of inductance 30, while in the embodiment of FIGS. 2A and 2B,these narrowings enable bringing spirals 21 and 23 closer symmetricallywith respect to second spiral 22. Other alternatives are possible. Forexample, its may be provided to bring the narrowed sections closer toexternal spiral 34 rather than to internal spiral 31. In this case,internal end 10 of the inductance is also brought closer to externalspiral 34.

According to another embodiment not shown, the inductance is said to be“symmetrical” and includes a crossing approximately equidistant from theends of the winding which are both outside of it. In the case of awinding with several spirals, there are then several crossings, eachcontact recovery running under a single spiral.

An advantage of the present invention is that it reduces the seriesresistance of the inductance with respect to a conventional inductance.On the one hand, by reducing the length of the contact recovery track,the capacitance between said track and the substrate is reduced. On theother hand, by narrowing the spirals, the capacitance between thewinding and the contact recovery is reduced.

Another advantage of the present invention is that it decreases theintegrated circuit surface area in which the inductance is inscribed.Indeed, by bringing the spirals closer towards the inside at the levelof their narrowing, external connection pad 13 is brought closer to thecenter of the winding. This advantage especially appears from FIG. 3A.

Of course, the present invention is likely to have various alterations,modifications, and improvements which will readily occur to thoseskilled in the art. In particular, the respective sizing of theinductance sections, of their narrowing and of the underlying contactrecovery track depend on the application and are to be adapted case bycase by those skilled in the art. Further, the contact recovery levelmay be formed by any metallization or polysilicon level, or even by asubstrate region. Finally, the winding itself may include severalconductive levels in parallel (connected by vias) provided that thewinding has, at least in one of these levels, a resistance per unitlength smaller than that of the contact recovery.

What is claimed is:
 1. An integrated inductance, formed above asemiconductor substrate supporting a conductive contact recovery track,comprising: a flat winding having a turn and positioned above thesubstrate, in which the winding has a first winding portion that crossesabove the contact recovery track, wherein the winding portion above saidcontact recovery track has a width that is reduced compared to a secondwinding portion that is not directly above the contact recovery track.2. The inductance of claim 1, wherein the contact recovery trackcontacts and extends from an internal end of the winding to an externalpad.
 3. The inductance of claim 1, wherein the winding is conductive andhas a resistance per square that is substantially smaller than aresistance per square of the contact recovery track, the winding havinga thickness substantially greater than a thickness of the contactrecovery track.
 4. The inductance of claim 1, wherein the windingincludes a third winding portion that crosses above the contact recoverytrack and is separated from the first winding portion by a firstinterval, and a fourth winding portion that is not directly above thecontact recovery track and is separated from the second winding portionby a second interval that is greater than the first interval.
 5. Theinductance of claim 1, wherein the first winding portion has a length,which depends on the width of the contact recovery track, that is chosento be as short as possible.
 6. The inductance of claim 1, wherein thewinding and the contact recovery track are made of aluminum.
 7. Theinductance of claim 1, wherein the winding is copper having a thicknessof several tens of micrometers, the contact recovery track being made ofaluminum of a thickness on the order of one micrometer.
 8. Theinductance of claim 1, wherein said contact recovery track is part of aconductive level formed by an upper metallization level used forinterconnections of other components of an integrated circuit formed inthe substrate.
 9. The inductance of claim 1 wherein the winding includesa plurality of concentric spirals having respective first windingportions that respectively cross over the contact recovery track andremainders that do not cross the contact recovery track, each firstwinding portion having a reduced width compared to the remainder of thespiral.
 10. An integrated inductance, formed above a semiconductorsubstrate that supports a conductive contact recovery track, comprisinga winding of several turns, in which the winding includes first andsecond winding portions that each cross the contact recovery track andare separated from each other by a first interval, wherein at least oneof the first and second winding portions is reduced compared to a thirdwinding portion that does not cross the contact recovery track or thefirst interval is reduced compared to intervals between portions of thewinding that do not cross the contact recovery track.
 11. The inductanceof claim 10, wherein the winding has an external turn with a turnportion adjacent to the contact recovery track that is closer to acenter of the winding than is the rest of the external turn.
 12. Aninductance device, comprising: a semiconductor substrate; a conductivecontact recovery track supported by the substrate and having first andsecond ends; an insulating layer on the contact recovery track; and aconductive winding, on the insulating layer, having an internal end andan external end, the internal end being electrically connected to thefirst end of the contact recovery track, the winding having a windingsection with a first width that is directly above the contact recoverytrack and a second width that is not directly above the contact recoverytrack, the first width being smaller than the second width.
 13. Theinductance device of claim 12, wherein the contact recovery trackextends from the internal end of the winding to an external pad that isexternal to the winding.
 14. The inductance device of claim 12, whereinthe winding has a resistance per square that is substantially smallerthan a resistance per square of the contact recovery track, the windinghaving a thickness substantially greater than a thickness of the contactrecovery track.
 15. The inductance of claim 12, wherein the windingsection includes first, second, third, and fourth winding portions, thefirst and second winding portions being above and crossing the contactrecovery track and being separated from each other by a first interval;the third and fourth winding portions being positioned not directlyabove the contact recovery track and being separated from each other bya second interval that is greater than the first interval.
 16. Theinductance of claim 12, wherein the winding section includes first,second, third, and fourth winding portions, the first and second windingportions being above and crossing the contact recovery track and beingseparated from each other by a first interval; the third and fourthwinding portions being positioned not directly above the contactrecovery track and being separated from each other by a second interval,the first and second winding portions being narrower than the third andfourth winding portions.
 17. The inductance device of claim 12, whereinthe first width of the winding section is substantially constant acrossthe contact recovery track and the winding section has an increasedwidth beginning immediately adjacent to the portion of the windingsection directly above the contact recovery track.
 18. The inductancedevice of claim 12, wherein the first and second ends of the contactrecovery track are separated by a distance that is less than the secondwidth of the winding section.
 19. The inductance device of claim 12,wherein the winding includes a plurality of concentric spirals, each ofthe spirals having a reduced width directly above the contact recoverytrack compared to the remainder of the spiral.
 20. The inductance ofclaim 12 wherein the winding includes a plurality of concentric spiralshaving respective first winding portions that respectively cross overthe contact recovery track and remainders that do not cross the contactrecovery track, each first winding portion having a reduced widthcompared to the remainder of the spiral.